Gas turbines such as those used to drive electric power generators have a number of rotor discs axially spaced along a rotor shaft to form interstage disc cavities. Designs for these components are varied. See, for example, U.S. Pat. Nos. 7,052,240 and 6,668,114 each incorporated herein by reference. Generally, the stages of the stator extend radially inward from the turbine casing into the interstage disc cavities. Each stator stage includes a number of stator vanes secured to the turbine casing and a seal assembly which seals against the rotor discs to prevent main gas flow from bypassing the vanes.
The combination of each stator section with the upstream and downstream rotor discs forms annular disc cavities. Cooling air bled from the compressor is introduced into the interstage disc cavities to cool and purge the seal assemblies. Typically, the cooling air flows axially and radially outward through the disc cavities and passes outward through a rim seal into the main gas flow.
Despite the provision of the rim seal and an adjoining rim seal cavity about the exit of the disc cavity, it is common for some of the main gas flow to at times ingress into the disc cavities. For example, pressure variations induced by the rotating parts may cause recirculation of gases within the cavities, and this can draw the very hot main gas flow toward the stator, rendering components vulnerable to thermal damage. Sufficient cooling gas must be provided in order to protect the rotor seals from the hot main gas ingress. This reduces the overall efficiency of the gas turbine.
There is a need, therefore, for an improved interstage disc cavity design in a gas turbine which provides greater protection from thermal damage and which results in improved operating efficiency. More particularly, there is a need for a reduction in the volume of cooling air needed to cool components in the interstage disc cavities of a gas turbine. It is desirable that such a design will reduce the amount of heating which may occur within the interstage disc cavities of a gas turbine due to ingress of main gas flow.